56 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



other constituent minerals by their form or optical characters . Quartz 

 can generally be distinguished from the triclinic felspars by the 

 compound twin striation of the latter, and from orthoclase by the 

 difference in general shape. Quartz, having no cleavage, breaks 

 into grains of most irregular form, vrhereas orthoclase, having 

 well-pronounced cleavage, very often occurs in flat pieces showing 

 more or less of straight parallel edges ; it also very often differs 

 in being more or less opaque from partial decomposition. How- 

 ever, when very transparent and the cleavage indistinct, as it is 

 sometimes in the glassy felspar of volcanic rocks, it requires much 

 care to distinguish it from quartz, since the form of the fragments 

 and their action on polarized light are very similar. When 

 neither fluid-cavities nor other enclosures are present to indicate 

 which of the two minerals is under examination, the only distinc- 

 tion that I have been able to discover depends on the slightly less 

 refractive power of glassy felspar, which causes the outline of the 

 fragments to be more distinct when mounted in hard Canada 

 balsam than that of quartz grains, which have almost exactly the 

 same refractive powder as hard balsam. It would, however, scarcely 

 be in place on the present occasion to describe in detail all the 

 characters which enable an observer to identify every constituent 

 mineral, though it was, as I think, desirable to describe some of 

 the more important, in order that a better opinion might be formed 

 respecting the amount of confidence that should be placed in my 

 general results. 



Identification of Fragments of Rocks in Sand. 



The difliculty of determining the true nature of each particular 

 grain is greatly increased when we have to deal with fragments of 

 rocks. This of course differs as much from studying the structure 

 of a known rock, as the determination of the true nature of any 

 single unknown bone differs from the study of the entire skeleton 

 of some well-known animal ; and in like manner it is necessary to 

 pay attention to matters of detail w^hich otherwise might be over- 

 looked. The grains also may be, and often are, too small to show 

 such characters as would suffice to identify the rock on a larger 

 scale ; and separate fragments may differ as much as though they 

 were different rocks, and yet may have been closely associated in 

 the same rock. It is also sometimes difficult to distinguish between 

 such very different objects as grains of partially decomposed felspar, 

 decomposed felsite, or slates formed from similar material, especi- 

 ally when we can examine only loose fragments and not a thin 

 section. Still, notwithstanding all these difficulties, to which we 

 should be fully alive, by one means or other very much may be 

 learned, even though in many cases the conclusions may be open to 

 a certain amount of doubt in some particulars. 



Flint Sand. 

 At one time I fully expected to find that the sand associated with 



